D-lactic acid has recently attracted attention as a raw material of a stereocomplex-type polylactic acid with L-lactic acid, or as a pharmaceutical intermediate. In either of the above uses, D-lactic acid as a raw material is required to have high optical purity.
Lactic acid is industrially produced by generally a fermentation method using a microorganism. A fermentation process is a process whereby a microorganism is cultured and substance production is carried out using a grown microorganism as a catalyst. Thus, the growth of a microorganism requires a source of nutrients. Corn steep liquor, which is inexpensive and often used as a source of nutrients in industry, is a nutritious liquid obtained in a process of processing corn and containing amino acids and the like at high contents. However, corn steep liquor, containing both of L-lactic acid and D-lactic acid, is one of factors that cause a reduction in optical purity.
From such a view point, for the purposes of efficient production of lactic acid and improvement in optical purity of a product, investigations have been conducted about use of raw materials not containing L-lactic acid, a reduction of the amount of L-lactic acid from raw materials or products, and the like.
For example, JP-A No. 2005-528111 employs a raw material containing no L-lactic acid and uses an oxygen uptake rate per specific microorganism as a control parameter so as to carry out adjustment such that a desired oxygen uptake rate per microorganism is maintained in a production phase. It has been reported that, as a result of the above, a high production speed of lactic acid and a high yield of lactic acid can be obtained.
JP-A No. 2008-301766 has reported that high yield of lactic acid can be obtained by employing a raw material containing no L-lactic acid and adding folic acid to a culture medium for improvement in the yield of lactic acid.
The pamphlet of WO 2005/033324 describes that L-lactic acid contained in a raw material is efficiently decomposed by using a microorganism that is modified so as to produce D-lactic acid highly selectively with high yield, thereby improving optical purity and improving the D-lactic acid productivity. It is also described that, although aeration may be completely omitted, cultivation may be performed at normal pressure, an aeration rate of from 0.1 vvm to 1 vvm, and a stirring rate of from 50 rpm to 500 rpm, that is, in the case of water at a temperature of 30° C., under aeration and stirring conditions in which an oxygen transfer rate coefficient (kLa) at normal pressure is from 1/hr to 400/hr.
Such improvement in productivity by microorganism modification and raw material adjustment has an advantage in that existing equipment can be used as it is. However, it is necessary to investigate a biological activity of a lactic acid-producing microorganism, and development of a new microorganism reaction system may take time.
Meanwhile, JP-A No. 7-313174 discloses a method for processing a substrate at an applied pressure of at least 1 MPa, for example, under a condition of 100 MPa or 10 MPa, as an enzyme reaction method capable of suppressing the growth of microorganisms, such as bacteria, during reaction without reducing enzymatic activities.
JP-A No. 2002-78495 discloses an enzyme processing method capable of efficiently converting a substrate to obtain an enzyme reaction product, as an enzyme processing method capable of efficiently performing enzyme decomposition processing in a short time. It is described that this method activates an enzyme by pressure application and also improves the stability of the enzyme itself, thereby allowing for processing at a higher temperature than a normal enzyme reaction temperature. For example, enzyme processing under a condition of 150 MPa is disclosed.